AN MD80 PLT RPTS AN UNDESIRED AND UNCOMMANDED XFER OF FUEL FROM THE L MAIN TANK TO THE CTR TANK.

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FLT EXPERIENCED AN UNWANTED FUEL XFER FROM THE L MAIN TANK TO THE CTR TANK. THIS FLT BEGAN WITH APPROX 2000 LBS OF CTR TANK FUEL; 20000 LBS TOTAL AT THE GATE. THE UNWANTED XFER DID NOT BEGIN UNTIL ALL THE CTR TANK FUEL WAS CONSUMED AT TOP OF CLB. AFTER USING ALL THE CTR TANK FUEL; I TURNED OFF THE CTR TANK PUMPS. AFTER A FEW MINS AT CRUISE; I NOTICED A FUEL IMBALANCE OF APPROX 500 LBS BETWEEN THE MAIN TANK INDICATION (L LOWER THAN THE R). ALSO; THERE WAS ABOUT 300 LBS IN THE CTR TANK AGAIN. AT THIS POINT; I WAS A LITTLE SUSPICIOUS ABOUT THE RETURN OF THE CTR TANK FUEL QUANTITY; HOWEVER; SOME XFER FROM THE ENGINE PUMPS TO THE CTR TANK IS NORMAL ON THE MD80. ALL OTHER PARAMETERS WERE NORMAL -- FUEL FLOW WAS THE SAME FOR BOTH ENGINES; AND TOTAL QUANTITY CHKED GOOD AT THE FIRST WAYPOINT. I TURNED THE CTR PUMPS ON ONCE AGAIN AND EMPTIED THE CTR TANK. WHEN THAT WAS ACCOMPLISHED; I BALANCED THE FUEL QUANTITY BY OPENING AND CROSS-FEED VALVE USING R MAIN TANK FUEL UNTIL THE QUANTITIES WERE BALANCED AGAIN. I WATCHED THE FUEL QUANTITIES CAREFULLY AND NOTED THAT THE L TANK WAS DECREASING FASTER THAN THE R; AND FUEL WAS BEING ADDED TO THE CTR TANK AT THE RATE OF ABOUT 100 LBS PER MINUTE. AT THIS POINT; WE KNEW WE HAD AN UNCOMMANDED XFER OF FUEL; WHEN THE CTR TANK FUEL REACHED ABOUT 400 LBS; I TURNED THE CTR TANK PUMPS ON AND LEFT THEM ON FOR THE COMPLETION OF THE FLT. THIS CAUSED ANY FUEL THAT WAS BEING XFERRED FROM THE L TANK TO THE CTR TO BE IMMEDIATELY RETURNED TO THE ENGINE FOR CONSUMPTION. THE FUEL IMBALANCE STAYED WITHIN ABOUT 300 LBS FOR THE COMPLETION OF THE FLT. THE QRH CALLS FOR A LNDG AT THE NEAREST SUITABLE ARPT. BY THE TIME WE IDENTIFIED THE PROB; WE WERE WELL BEYOND THE HALF-WAY POINT TO OUR ORIGINAL DEST APCHING TOP OF DSCNT; AND I CHOSE IT AS THE SAFEST OPTION FOR OUR LNDG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED HE BELIEVES THE DE-FUELING VALVE MAY HAVE BEEN LEFT OPEN ALONG WITH THE CENTER OR LEFT WING TANK VALVE ALLOWING FOR FUEL TO BE PUMPED INTO THE CENTER TANK. COMPOUNDING THE ISSUE IS THE FACT THAT THE MD80 COCKPIT ANNUNCIATOR LIGHTS DO NOT INDICATE IF THE DE-FUELING VALVE OR ANY OF THE LEFT; RIGHT OR CENTER TANK FUEL FILL VALVES ARE STILL OPEN VIA THE WING FUELING PANEL SWITCHES. THE CROSS-FEED VALVE ONLY ALLOWS FOR FUEL TO BE USED FROM EITHER LEFT OR RIGHT TANKS; NO FUEL TRANSFERS IN FLT. HOWEVER; IF THE DE-FUELING VALVE IS OPEN WHICH CAN ONLY BE DONE FROM THE GROUND; THEN AN IN FLIGHT UNWANTED AND UNCOMMANDED FUEL TRANSFER CAN OCCUR. REPORTER ALSO ADDS; WITH THE SHORT AMOUNT OF TIME FOR MAINT TO 'FIX' THE PROBLEM AND WITHOUT ANY OTHER FUEL TRANSFER ISSUES AFTER; HE FEELS THE DE-FUELING VALVE BEING LEFT OPEN WAS MOST LIKELY THE ISSUE.